Oxia Island—or Sivriada—was often used by Byzantine clergy as a remote place for peaceful worship, and by Byzantine emperors as a convenient prison for holding prominent individuals they considered troublesome. The first known person imprisoned on the island by order of Emperor Nikephoros I was Platon Sakkudion, the uncle of the famous clergyman Theodoros Stoudites, for supporting his nephew in his conflict with the emperor. Other notable people who stayed on the island for religious and political reasons included Hebon, Basil Skleros, Patriarch John of Constantinople, and Patriarch Michael II of Constantinople. The graves of those who died on the island during the Byzantine period can still be seen today. The ruins of a Roman settlement and a ninth-century Byzantine monastery are still visible on the shore, near the fishermen’s shelter, a small pier frequently used by yachts. The most important buildings on the island were constructed in the ninth century AD, including a church, a chapel dedicated to religious martyrs, a monastery at the eastern tip (its walls are still visible), and a cistern in the center of the island (part of which can still be seen).

In 1073, John Doukas, brother of Emperor Constantine X, fled to the island, fearing retribution from Nikephoritsa—the regent of Michael VIII. However, Nikephoritsa herself soon fell out of favor and was also sent to Oxia in 1078, where she died from torture.

But mostly, people did not try to escape from this island; on the contrary, they sought by all means to reach it and seek unity with God. In Byzantine times, a house for orphans, the Church of Archangel Michael, and a monastery were built on the small island, from the walls of which several patriarchs emerged. Voluntarily, Patriarch Michael II Kourkouas of Constantinople went into exile on the island, who in 1146 realized that he was not suited to managing the church affairs of the empire, earning the respect of his contemporaries.

During the Ottoman Empire, the island became deserted, Byzantine buildings fell into ruin, and by order of the governor of Istanbul, all stray and unhealthy animals (from cats to horses) of the city were brought here by boats and left to their fate. By rough estimates, no fewer than 80,000 stray dogs alone were brought here.

The Princes’ Islands played an important role in the correctional system of the Byzantine Empire and were designed for the humane (as much as one can say) detention of political and religious opponents. Despite the absence of prisons as such, monasteries, numbering at least 14, perfectly fulfilled their function; most of them were destroyed as a result of punitive operations by the Turks during the Ottoman period. From the late 12th century, the Princes’ Islands were no longer used in Byzantium as a place of exile and imprisonment for influential persons, which was related to the general decline of the state. However, during the Ottoman Empire, for some time, they regained their former status for potential claimants to the sultan’s throne.

Sources:

https://en.wikipedia.org/wiki/Princes%27_Islands

https://cyberleninka.ru/article/n/printsevy-ostrova-v-penitentsiarnoy-sisteme-vizantiyskoy-imperii/viewer

https://dzen.ru/media/mir_v_ego_mnogoobrazii/princevy-ostrova-glavnaia-tiurma-vizantiiskoi-imperii-617d348c30951a5ab928632d